For a sorptive type filter, i.e., one which filters by adsorption or absorption, of particulate material, maximum efficiency and life span are attained when the sorptive particles are packed together in a bed. For a thin bed filter, i.e., from less than 1/2" to 2 or more inches thick, this can be obtained by simply filling the space between two spaced apart perforated sheets with loose carbon particles. Such filters, herein referred to as "filled filters", have been manufactured and sold by D-Mark, Inc. of Chesterfield Township, Mich. as well as by others. While resulting in a high capacity filter, the particles tend to settle resulting in channelling and shedding of the sorptive particle dust such as carbon dust.
Shedding and channelling is overcome as disclosed in U.S. Pat. No. 3,019,127 but only a very low carbon loading results, somewhere on the order of 4% of particulate material per unit volume of the web. Increased carbon loading, while avoiding shedding and channelling, is disclosed in U.S. Pat. No. 4,227,904 wherein carbon particles are glued to the face of a perforated substrate to provide a layer of particles on the substrate. Two such substrates are then placed together with the carbon covered faces in opposition and a border frame is secured about the edges to hold the substrates together. This results in a medium loaded product which has enjoyed substantial commercial success.
Finally, heavily loaded thin bed filters which avoid channelling and the other drawbacks of the prior art and methods of making them are disclosed in U.S. Pat. Nos. 5,124,177 and 5,338,340. These filters have a maximum loading of approximately 90-100 grams per square foot with a 3/8" thick mat, and up to 300 grams per square foot with a mat approximately 3/4" thick. These loadings have a very acceptable pressure drop, and with roll-coating, "driving" and/or rolling techniques, they have been able to achieve extremely good adhesion with a minimum of shedding during handling and final assembly. From a performance point of view, the carbon is not totally encapsulated during the manufacturing process, and therefore the vast majority is available for first pass absorption.
While such filters are enjoying commercial success, these products do not always contain a uniform loading of the particulate throughout the web or pad and the density or porosity may vary from pad to pad or throughout the same pad. This is not the fault of the methods disclosed in such patents, but rather the pad as received from the pad manufacturer varies. The prior art does not offer a solution that resolves the problem, and provides a uniform particulate loading in the pad. Accordingly, a different method of constructing the filter disclosed in U.S. Pat. No. 5,338,340 is needed, one that provides uniformity in loading and density of the finished filter, and which allows an increase in the loading.